Solvatochromic study of 2-hydroxy-4-methylquinoline for the determination of dipole moments and solute–solvent interactions

Solvatochromism and estimation of ground and excited state dipole moments of 6-aminoquinoline

The spectral behaviour of 6AQ was investigated using fluorescence spectroscopy in several polar and non-polar solvents. Both the absorption and fluorescence spectra displayed solvatochromism. The Stokes shift increased significantly with increasing solvent polarity and signifies a more polar excited state with possible change in the excited state (ES) geometry. The involvement of π→π^∗ transition was observed. The ground state (GS) and excited state (ES) dipole moments were determined by the solvatochromic shift method using Bilot-Kawaski, Lippert-Mataga, Kawski-Chamma-Viallet, and Reichardt equations. The experimental value of GS dipole moment matches closely with the theoretical value computed using DFT/B3LYP/6-311G(d,p). The ES dipole moment is higher than the GS dipole moment. Besides, the solvatochromic study reveals that the ES of 6AQ is more polarized than the GS due to intramolecular charge transfer (ICT), possibly aided by a change in the geometry of the molecule in the ES. The influence of the non-specific and specific interactions in the photophysical properties of the titled molecule was analyzed using the Catalan scale. The study shows that 6AQ has reasonable band-gap energy and good CIE chromaticity coordinate in the blue region close to the national television standard committee system (NTSC) for the ideal blue CIE coordinate. Therefore, future research into 6AQ as a source of light-emitting diodes and fluorescent sensors may have potential applications in the field of optoelectronics.

Solvatochromism and linear solvation energy relationship of the kinase inhibitor SKF86002

We studied the spectroscopic characteristics of SKF86002, an anti-inflammatory and tyrosine kinase inhibitor drug candidate. Two conformers SKF86002A and SKF86002B are separated by energy barriers of 19.68kJ·mol(-1) and 6.65kJ·mol(-1) due to H-bonds, and produce the three major UV-Vis absorption bands at 325nm, 260nm and 210nm in cyclohexane solutions. This environment-sensitive fluorophore exhibited emission in the 400-500nm range with a marked response to changes in environment polarity. By using twenty-two solvents for the solvatochromism study, it was noticed that solvent polarity, represented by dielectric constant, was well correlated with the emission wavelength maxima of SKF86002. Thus, the SKF86002 fluorescence peak red shifted in aprotic solvents from 397.5nm in cyclohexane to 436nm in DMSO. While the emission maximum in hydrogen donating solvents ranged from 420nm in t-butanol to 446nm in N-methylformamide. Employing Lippert-Mataga, Bakhshiev and Kawski models, we found that one linear correlation provided a satisfactory description of polarity effect of 18 solvents on the spectral changes of SKF86002 with R(2) values 0.78, 0.80 and 0.80, respectively. Additionally, the multicomponent linear regression analysis of Kamlet-Taft (R(2)=0.94) revealed that solvent acidity, basicity and polarity accounted for 31%, 24% and 45% of solvent effects on SKF86002 emission, respectively. While Catalán correlation (R(2)=0.92) revealed that solvatochromic change of SKF86002 emission was attributed to changes in solvent dipolarity (71%), solvent polarity (12%), solvent acidity (11%) and solvent basicity (6%). Plot of Reichardt transition energies and emission energies of SKF86002 in 18 solvents showed also a linear correlation with R(2)=0.90. The dipole moment difference between excited and ground state was calculated to be 3.4-3.5debye.

Experimental and theoretical study of hydroxyquinolines: hydroxyl group position dependent dipole moment and charge-separation in the photoexcited state leading to fluorescence

Optical absorption and fluorescence (FL) spectra of 2-, 6-, 7-, 8-hydroxyquinolines (2-,6-,7- and 8-HQs) have been measured at room temperature in the wide range of solvents of different polarities, dielectric constant and refractive index. The ground state dipole moment (µ _g) and excited state dipole moment (µ _e) of 2-, 6-, 7- and 8-HQs were obtained using solvatochromic shift (SS) methods and microscopic solvent polarity parameters (MSPP). Change in the dipole moment (Δµ) between the ground and photo-excited states was estimated from SS and MSPP methods. DFT and TDDFT based theoretical calculations were performed for the ground and excited states dipole moments, and for vertical transitions. A significant enhancement in the excited state dipole moment was observed following photo-excitation. The large value of Δµ clearly indicates to the charge-separation in the photo-excited states, which in turn depends on the position of the hydroxyl group in the ring.

Determination of the ground- and excited-state dipole moments of bromocresol purple in protic and aprotic solvents

Although it has been widely recognized that hydrogen bonds play a significant role in the photophysics of molecules, this phenomenon has rarely been applied to the solvatochromic method for determination of dipole moments. The difference in the dipole moment between the ground and excited state was determined in protic and aprotic solvents using both the Lippert-Mataga equation and the Bilot-Kawski equation for bromocresol purple, a molecule capable of hydrogen-bond donation and acceptance. The dipole change in protic environments was determined to be 15.2 ± 1.0 D for the Lippert-Mataga method and 9.2 ± 1.0 D for the Bilot-Kawski method, while the change in aprotic environments was 10.4 ± 1.0 D and 6.7 ± 1.0 D, respectively. Both methods highlighted the importance of hydrogen bonding in stabilizing increased charge-separation of the excited state, allowing for larger changes in dipole moments in protic environments. This study further validates a simple, rational modification to the commonly used methods that allows access to dipole-moment data on dyes with hydrogen-bonding capabilities through solvatochromic experiments.

4-alkoxyethoxy-N-octadecyl-1,8-naphthalimide fluorescent sensor for human serum albumin and other major blood proteins: design, synthesis and solvent effect

A series of 4-alkoxyethoxy-N-octadecyl-1,8-naphthalimides with intense blue fluorescence were designed and synthesized as polarity and spectrofluorimetric probes for the determination of proteins. In solvents of different polarities, the Stokes shifts of two dyes increased with increasing solvent polarity and fluorescence quantum yields decreased significantly, suggesting that electronic transiting from ground to excited states was π-π(*) in character. Dipole moment changes were estimated from solvent-dependent Stokes shift data using a solvatochromic method based on bulk solvent polarity functions and the microscopic solvent polarity parameter (E(T)N). These results were generally consistent with semi-empirical molecular orbital calculations and were found to be quite reliable based on the fact that the correlation of the solvatochromic Stokes shifts with E(T)N was superior to that obtained using bulk solvent polarity functions. Fluorescence data revealed that the fluorescence quenching of human serum albumin (HSA) by dyes was the result of the formation of a Dye-HSA complex. The method was applied to the determination of total proteins (HSA + immunoglobulins) in human serum samples and results were in good agreement with those reported by the research institute.

Excited state electric dipole moment of nile blue and brilliant cresyl blue: a comparative study

A solvatochromic study on the photophysical properties of two cationic oxazine dyes (brilliant cresyl blue and nile blue) was carried out. The electronic absorption and emission spectra of the dyes were recorded in various organic solvents with different polarity. The ground and the excited state dipole moments of the dyes were estimated from solvatochromic shift method. The solvent dependent spectral shifts in absorption and fluorescence spectra were analyzed by the Katritzky and Kamlet-Taft multi-parameter scales. This work is characterized by detailed quantitative studies on the nature and extent of solvent-solute interactions.